The present invention relates to a hip joint endoprosthesis having an adjustable prothesis head that is connected with a stem that can be fixed in the femur. The prosthesis head includes an adjusting device, which can be fixed by locking devices, and has a link element guided in a stationary link area. A supporting body of a joint head is held on the link element and is supported to be adjustable with respect to the link area via an intermediate plate. The locking devices are adjustable with respect to one another via a tension element which penetrates the adjusting device.
A hip joint endoprosthesis comprising an adjustable prosthesis head which is connected with a stem which can be fixed in the femur is shown in German Patent Application P 41 27 989.1. The prosthesis head is constructed in several pieces and is adjustable in three degrees of freedom. Toothings provide a finely graduated adjustability of a displacement in the medial-lateral direction and of a rotation around an axis perpendicularly to this direction. Securing takes place by means of a single screw which holds the meshing toothings in a tensioned state with respect to one another. The head of the joint is placed on the prosthesis head via a cone. An adjustment of the height of the head of the joint takes place by means of joint heads with cones which are pressed in deeply to different degrees and cause a displacement of the center point of the joint head in the proximal-distal direction.
An object of the invention is to provide an improved adjustable hip joint endoprosthesis which, in the case of an overload, prevents damage to the part of a hip joint endoprosthesis which is implanted in the femur.
This and other objects are achieved by the present invention which provides a stem fixable to a femur, and a prosthesis head connected to the stem. The prosthesis head includes an adjusting device, locking devices that fix the adjusting device, and a link element guided in a stationary link area. A joint head, having a supporting body, is held on the link element and is adjustable with respect to the link area via an intermediate plate. A tension element penetrates the adjusting device and fixes the locking devices with respect to one another. The prosthesis head has a force limiting device that limits to a maximum value a force to be transmitted by the hip joint endoprosthesis.
By means of the invention, a hip joint endoprosthesis is advantageously provided which, in the case of an overload, prevents damage to the part of a hip joint endoprosthesis which is implanted in the femur and, at the same time, also prevents a resulting damage to the femur.
The limiting of the forces takes place by the deformation or fracture of a material cross-section or the sliding of a frictionally engaged element in the prosthesis head. In the event of a deformation or a sliding, the prosthesis can still be used for a short period of time. As a result, a new implantation, which is connected with high risks, is avoided in each case. Instead, only that part of the prosthesis head must be exchanged which, as the force limiting device, has absorbed that portion of the force by fracture or deformation which exceeds a predetermined maximum value. When the force limiting device is constructed as a frictionally engaged element, an exchange is not even necessary but only a new adjustment for the restoring of the geometry of the hip joint endoprosthesis.
As a result of the deformation or fracture of a material cross-section or a sliding of a frictionally engaged element, an overloading is rendered clearly visible and noticeable by an extreme faulty position of the thigh.
A case of overload is characterized by the exceeding of the maximum value of the force that can be transmitted from the hip joint endoprosthesis to the femur. The maximum value of the force which can be transmitted by the force limiting device is selected in such a manner that the overload situation will not be reached, but that forces occurring in everyday life are below it. A minor exceeding of the maximum value, as may occur in exceptional situations, in the case of the material normally used for the prosthesis and an impulse-type duration of the effect, does not lead to a response of the force limiting device but is transmitted. By contrast, an exceeding of the maximum value in the event of an overload case must result in an immediate response of the force limiting device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.